1. Field of the Invention
The present invention is related to hardeners for thermosettable compositions; and more specifically, to a multifunctional aromatic amine useful as a curing agent for a thermoset resin such as an epoxy resin; and a process for preparing the thermosettable compositions.
The thermosettable compositions of the present invention are useful in various applications such as electrical, electronic, casting, potting, encapsulation, and composites.
2. Description of Background and Related Art
It is known to use thermosetting or thermoset resins such as epoxy resins in combination with curing agents to form thermoset cured products useful in various fields such as, for example, in the field of composites, electrical laminates and coatings. For some applications such as for manufacturing composites, electrical laminates and coatings, it is required to prepare and use a thermoset product with a high heat resistance (e.g. a glass transition temperature (Tg) of greater than 200° C.) in order for the thermoset product to be effective when used in a high temperature environment.
Aromatic amine hardeners commonly used in preparing thermosettable compositions include for example, methylene dianiline (also known as 4,4′-methanediyldianiline) [MDA], toluene diamine (TDA) and various alkylated derivatives. These known hardeners (also referred to as curing agents or crosslinking agents) impart a moderately high Tg value in the cured polymer. For example, the use of MDA with bisphenol A diglycidyl ether provides a thermoset with a Tg of 187° C., and the use of MDA with an epoxy novolac (e.g., D.E.N.™ 438) provides a thermoset with a Tg of 206° C. However, these known hardener materials have some volatility and undesirable toxicological characteristics that limit their use in some cases.
Polyaromatic amine hardeners derived from aromatic dialdehydes are also known (see e.g. Kanayama JP62227920 Chem. Abs. Ref. 108:132850). When polyaromatic amine hardeners derived from aromatic dialdehydes are used as curing agents with epoxy resins, heat distortion temperatures of as high as 286° C. are obtained compared to 167° C. using MDA. Heat distortion temperatures are typically within 20° C. (+/−) of the Tg, and therefore such hardeners would meet the needs of the industry. Unfortunately, the precursors for the polyaromatic amine hardeners, such as benzenedialdehydes, are difficult and expensive to prepare. For example, one common method to prepare the polyaromatic amine hardeners is to react benzaldehyde with chloroform in the presence of a strong base. This gives a mixture of bis-(dichloromethyl)benzaldehydes that can be converted to the dialdehydes with aqueous KOH (see e.g. Chaudhuri, J. Amer. Chem. Soc., 64, p. 315 (1942)). Other methods for preparing dialdehydes are described in U.S. Pat. No. 4,845,305. The overall yields of the dialdehydes from commercially available starting materials using these multi-step processes are unsatisfactory, and give numerous side-products. As a consequence, the benzene dialdehyde isomers (phthalaldehyde, isophthalaldehyde, and terephthalaldehyde) are only available as specialty chemicals and are quite expensive.
It is desired to provide a multifunctional aromatic amine hardener composition for thermoset resins such as epoxy resins which can impart a high Tg to the resulting cured thermoset product after cure and which can be prepared inexpensively.